The specific aims of this protocol are 1) to characterize the longitudinal changes in maternal carbohydrate metabolism in obese control subjects and obese women with gestational diabetes prior to conception and in early and late gestation and 2) to determine if the changes in maternal energy expenditure and body composition in early gestation are related to alterations in maternal carbohydrate metabolism. The aims will be evaluated using: 1) the oral glucose tolerance test, 2) intravenous glucose tolerance test, 3) hydrodensitometry, 4) a low and high dose hyperinsulinemic-euglycemic clamp, 5) indirect calorimetry; and 6) doubly labeled water.